Technical Field of the Invention
The present invention relates to systems that can store and release energy using hygroscopic materials. Specifically, systems based on hygroscopic materials can be selectively exposed to high or low humidity environments in order to cause the materials to expand or contract to do useful work as well as store and release energy.
Description of the Prior Art
Natural evaporation across open water facilitates the energy exchange between oceans and atmosphere, thereby fueling the winds and warm weather on earth. Under dry atmospheric conditions evaporation can be harnessed to do useful work, for example, the tree uses evaporation to transport water from soil to the leaves. Plants also use swelling and shrinking of cell walls for mechanical actuation. These processes have inspired novel approaches to engineering actuators, pumps, biological sensors and even energy scavengers to power micro- and nano-devices. In principle, evaporation has the potential to become a significant source of renewable energy. However, this requires useful work to be generated from evaporation with high efficiency, high power levels, long term sustained performance, and without consuming fresh water.
Bacterial spores are dormant cells that can withstand harsh environmental conditions for long periods of time and still maintain biological functionality (FIG. 1a). Despite their dormancy, spores are remarkably dynamic structures. For example, Bacillus spores respond to changes in relative humidity (RH) by expanding and shrinking anisotropically and changing their diameter by as much as 12% (FIG. 1b). The density of fully hydrated and expanded spores are significantly lower than dry spores; ˜1.2 g/ml vs. ˜1.5 g/ml for B. subtilis. The reduction of mass density despite absorption of additional material requires spores to expand their volume highly efficiently.